I am looking for books that attempt a dispassionate answer to the
question "What has Christianity's overall effect been on history?" On
the negative side would be, for instance, the Spanish Inquisition, and
on the
positive side would be the building of hospitals in poor countries
etc. I am not looking for books on the truth or falsehoods behind
Christianity. The books should examine the broad strokes of history
and be intended for the educated non-specialist.

You might be interested in Linda Woodhead's "An Introduction to Christianity": 

"An Introduction to Christianity examines the key figures, events and
ideas of two thousand years of Christian history and places them in
context. It considers the religion in its material as well as its
spiritual dimensions and explores its interactions with wider society
such as money, politics, force, gender and the family, and
non-Christian cultures and societies."

http://www.chbookshop.co.uk/product.asp?id=2381839